The invention relates to the specific field of particularly thin and light mounts for spectacles made from plastics material and having a metal frame inserted completely within the front frame of the mount. Typically, a frame of different material is inserted within the front frame of the mount for ornamental and decorative purposes, in order to seek particular aesthetic effects, which may in particular be obtained by combining the transparency of the plastics material with the possible colours/decorations of the internal metal frame. Moreover, in the case of particularly thin plastics mounts, an internal frame, especially if metallic, is generally needed to make the front frame of the mount robust enough.
Stylistic and dimensional choices (for instance shape, cross-section, thickness) for this type of mount must therefore be such that they do not pose any problem when mounting the lens and are therefore somewhat limited, as will be explained in greater detail below.
For instance, if the front frame is produced using a plastics material which does not tend to soften under the effect of heating, the insertion of the lens should be carried out by means of the same method generally used for front frames of plastics materials which have no internal metal frame, i.e. by resilient deformation of the front frame.
In the case of this particular type of spectacles, it may be that the internal metal frame, even though thin, as it is substantially rigid, greatly reduces the resilient deformability of the front frame, thereby making the mounting of the lens more problematic.
For other types of plastics materials which tend more readily to soften under the effect of heating, the lens may be inserted in the front frame by means of the temporary, more or less localized, softening of the plastics material using an appropriate source of heat (as is generally also the case with mounts with front frames which have no internal metal frame).
The temporary yielding of the material of the front frame (in the parts designed to house the lens) obtained by heating, makes it easier to introduce the lens into its seat in the front frame.
However, in the case, for instance, of cellulose acetate, which is a plastics material used widely in spectacle production and which belongs to the group of materials which more readily soften as a result of heating, the operation to heat a front frame could be detrimental to the integrity of the front frame.
In the case of acetate front frames with very small thicknesses/cross-sections which are, for that reason, particularly appreciated from an aesthetic point of view, the softening of the material by heating, so that the lens may be readily mounted, may in practice be critical, as it may accidentally cause, in the parts of the front frame involved, changes in shape and/or dimensional variations which may be detrimental to the possibility of mounting the lens, or even the general retention of the “design” originally planned for the mount, leading to serious product defects which may well make the product impossible to sell or use.
In the spectacle industry, some plastics materials (often including cellulose acetate itself) are often processed in sheet form. In this case, a front frame of the above-mentioned type may be produced by a method of stratification of the plastics material on a metal frame (i.e. the application of at least one sheet of plastics materials to each of the front and rear surfaces of the frame, the assembly being made rigid by adhesion or like fastening systems). In accordance with this method of manufacture of the mount, the heating may nevertheless cause the problem of “delamination” in the front frame, i.e. the separation of at least one layer of plastics material from the metal frame to which it has previously been fastened, thereby damaging the frame or causing it to become defective.